#define _CRT_SECURE_NO_WARNINGS 1

#include<stdio.h>

int FindSingle(int arr[], int size)
{
	int result = 0;
	for (int i = 0; i < size; i++)
	{
		result ^= arr[i];
	}
	return result;
}
int main()
{
	int arr[100];
	int count = 0;
	int num;

	while (scanf("%d", &num) != EOF && count < 100)
	{
		arr[count++] = num;
	}

	int singleNumber = FindSingle(arr, count);
	printf("%d", singleNumber);
	return 0;
}
